rebase: use mboxrd format to avoid split errors

The mboxrd format allows the use of embedded "From " lines in
commit messages without being misinterpreted by mailsplit

Reported-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--am.sh

@@ -53,6 +53,7 @@ else
 
 	git format-patch -k --stdout --full-index --cherry-pick --right-only \
 		--src-prefix=a/ --dst-prefix=b/ --no-renames --no-cover-letter \
+		--pretty=mboxrd \
 		$git_format_patch_opt \
 		"$revisions" ${restrict_revision+^$restrict_revision} \
 		>"$GIT_DIR/rebased-patches"
@@ -83,6 +84,7 @@ else
 	fi
 
 	git am $git_am_opt --rebasing --resolvemsg="$resolvemsg" \
+		--patch-format=mboxrd \
 		$allow_rerere_autoupdate \
 		${gpg_sign_opt:+"$gpg_sign_opt"} <"$GIT_DIR/rebased-patches"
 	ret=$?

t/t3400-rebase.sh

@@ -255,4 +255,26 @@ test_expect_success 'rebase commit with an ancient timestamp' '
 	grep "author .* 34567 +0600$" actual
 '
 
+test_expect_success 'rebase with "From " line in commit message' '
+	git checkout -b preserve-from master~1 &&
+	cat >From_.msg <<EOF &&
+Somebody embedded an mbox in a commit message
+
+This is from so-and-so:
+
+From a@b Mon Sep 17 00:00:00 2001
+From: John Doe <nobody@example.com>
+Date: Sat, 11 Nov 2017 00:00:00 +0000
+Subject: not this message
+
+something
+EOF
+	>From_ &&
+	git add From_ &&
+	git commit -F From_.msg &&
+	git rebase master &&
+	git log -1 --pretty=format:%B >out &&
+	test_cmp From_.msg out
+'
+
 test_done